The counter bend is a great exercise to help the horse become supple and through. Suppleness comes from lateral, throughness from longitudinal. By asking for the counter bend I make my horse more responsive to the inside leg. The biggest mistake made in this exercise is to use too much inside hand and try to pull the horse over with the hand, rather than invite him to move the leg. Correctly done the inside hand (the one to the inside of the bend) should not change when the direction changes. The dominant aids would be inside leg and outside hand.  More counter bending also in the Circle III exercise.